pthread: do not touch has_b_frames

pthread: do not touch has_b_frames

Adding the thread count in frame level multithreading to has_b_frames
as an additional delay causes more problems than it solves.
For example inconsistent behaviour during timestamp calculation in
libavformat.
Thread count and frame level multithreading are both set by the user.
If the additional delay caused by frame level multithreading needs
to be considered in the calling code it has all information to take
it into account.
Should it become necessary to calculate a maximum delay inside
libavcodec it should be exported as its own field and not reusing
an existing field.

Based on a patch by Michael Niedermayer.

Signed-off-by: Janne Grunau <janne-libav@jannau.net>